1. Field of the Invention
The present invention relates to a ceramic envelope consisting of a light transmittable ceramic employed for a high intensity discharge lamp, such as a high intensity sodium lamp or a metal halide lamp, and particularly relates to a ceramic envelope for high intensity discharge lamp formed by integrally forming electrode insertion section and at least end portions of a barrel section forming a discharge space.
2. Description of the Related Art
A discharge lamp employing a ceramic envelope made of light transmittable ceramics is advantageous in lamp efficiency, especially when the lamp has a discharge space formed into an elliptical shape by widening the diameter of a central section. Due to this, the discharge lamp is used in applications in which lamp efficiency is regarded as particularly important. A ceramic envelope of this type has the structure shown in FIG. 5, i.e., the ceramic envelope is formed such that an elliptical shape of barrel section 10 forming a discharge space is provided at the center thereof. In addition, capillary tubes 11 serving as electrode insertion sections are integrally formed with both end portions of the barrel section 10 in a longitudinal direction, while the capillary tubes 11 face each other. After light emission materials or starting gas is filled into the discharge space, electrodes are inserted into capillary tubes 11, and then capillary tubes 11 are sealed to form a discharge lamp.
The ceramic envelope of such a shape is manufactured by firing a member formed by casting, blowing, or vacuum molding techniques.
In case of the ceramic envelope having the integrally formed barrel section 10 and the capillary tubes 11, each of the internal boundary end portions 12 between the barrel section 10 and the capillary tubes 11 have a relatively large radius R because of its manufacturing technique. Due to this, when the electrode is inserted and the lamp is assembled, a large space is formed between the electrode and the ceramic envelope. While the lamp is lighted up, the temperature of this space is relatively low since the space is distant from the discharge section. As a result, the light emission materials in this space cannot be transformed into the materials in a gaseous phase but remain in the space in a liquid phase. This often causes an emission color change because the ratio of the light emission materials in the lamp changes thereby. In addition, these light emission materials, which are kept in the liquid phase, are exposed to high temperature while the lamp is lighted up. As a result, the ceramic envelope is gradually corroded and the extended service life of the lamp is impaired.
Moreover, since the linear transmittance of the ceramic envelope depends on surface roughness Rmax, it is more advantageous to set the surface roughness Rmax as small as possible. Nevertheless, while polishing can control the surface roughness of the inner surface of the ceramic envelope, the polishing step is complicated and not practical. Further, if the envelope is polished, an additive such as MgO or La2O3, which are considered to be less resistant against halide than alumina, appears on the inner surface of the envelope. Thus, good discharge characteristics cannot be maintained.
The present invention has been made in light of the above-stated problems. It is, therefore, an object of the present invention to provide a ceramic envelope for a high intensity discharge lamp formed by integrally forming electrode insertion sections and at least end portions of a barrel section. This envelope is capable of reducing light color changes of the high intensity lamp, and capable of extending the service life of the lamp.
According to a first aspect of the present invention, there is provided a ceramic envelope formed by integrally forming at least end portions of a barrel section forming a discharge space and small-diameter electrode insertion sections, which are provided to protrude outward from the end portions of the barrel section, respectively. The entire ceramic envelope is formed out of light transmittable ceramics. Further, a radius of curvature R is provided inside of a boundary end portion between the barrel section and at least one of said electrode insertion sections. A value of the R is set in a range of 0.01 to 3.0 mm.
By specifying the value of the R provided on the corrodible boundary end portion between the barrel section and each electrode insertion section to be small, the quantity of remaining light emission materials in a liquid phase can be decreased. Thus, the service life of the lamp is extended and the change of light color is reduced. In this case, it is more preferable that the value of R is in a range of 0.1 to 1.0 mm. A light emission tube having such value of R can be manufactured by lost wax techniques, powder press molding, extrusion press molding, frost molding, injection molding or gel casting molding techniques.
According to a second aspect of the present invention, there is provided a ceramic envelope for high discharge lamp according to the first aspect of the invention, wherein a surface roughness Rmax of an inner surface of the barrel section is set in a range of 0.01 xcexcm to 0.4 xcexcm, and the density of an additive on the inner surface of the barrel section is not more than half the density of the additive in a thick central portion of the barrel section.
By setting the Rmax value as the above, a barrel section with a good light transmittable property can be provided, and the reaction of the light emission materials such as halide with the additive can be suppressed. As a result, good discharge characteristics are maintained. More preferably, the surface roughness Rmax is in a range of 0.01 xcexcm to 0.1 xcexcm. It is noted that the density of the additive added to the ceramic on the surface of the envelope is reduced by the diffusion or splash action in the step of firing the formed member of the light emission tube. With this characteristic, the density of the additive on the surface can be set to be not more than half the density of the additive in the thick inside portion thereof. Due to this thin density of the additive, light emission materials such as halide can be suppressed from reacting with the additive and good discharge characteristics can be maintained.
According to a third aspect of the present invention, there is provided a ceramic envelope according to the second aspect of the invention, wherein the additive includes at least one material selected from the group consisting of Sc2O3, MgO, ZrO2, Y2O3 and a lanthanoid-based rare earth oxide.
By adding this additive, grains of the parent phase of ceramic represented by alumina are suppressed from abnormally growing, and thus uniform grain growth is induced. Moreover, the surface roughness Rmax is controlled to have an appropriate value.